Splatoon's co-director Tsubasa Sakaguchi says he has no plans for paid Splatoon DLC.
First up, you can expect more free content updates throughout summer (and beyond), as Sakaguchi stated that "beyond August, there'll be new stages and new weapons slowly being released." One thing you shouldn't expect is paid DLC content, with Sakaguchi stressing that "At the moment, we're not planning any paid download content."
This statement, of course, may not stand true for the future, and we may see some paid Splatoon content at some point later down the line. But at least for now we'll all be riding the free content train. Hooray!
Sakaguchi also talked a bit about what surprised him most when the game went live. "When we were designing Splatoon, reaching level 20 was going to take quite a lot of time," he mused. "Learning the game mechanics with the gyro sensor - we thought that would take a lot of time. In that sense we're surprised that people have reached the level cap in such a short time. We're actually really worried about those people - worried that they're still sleeping and eating!"
Never underestimate hardcore gamers.